The present invention relates to a gas permeable probe for use in an optical analyzer for an exhaust gas stream flowing through a duct or chimney, the probe comprising:                an elongate hollow structure having first and second ends and a side wall, with an optical cavity defined between the first and second ends within the side wall,        a filter forming part of the elongate hollow structure,        a mounting structure at the first end and adapted for mounting the elongate hollow structure within the duct or chimney,        an optical window at the first end permitting a light beam originating from an optical analyzer to enter into the optical cavity to travel from the first end to the second end, and        a retroreflector provided at the second end for returning the light beam to the first end of the hollow structure.        
A gas permeable probe of this kind is known, for example, from U.S. Pat. No. 4,560,873. The gas permeable probe disclosed in this reference utilizes a cylindrical ceramic filter to permit gas flowing through the chimney to enter into the optical cavity, with the pores of the filter being sized such that particulate material in the chimney is prevented from entering the optical cavity. A similar gas permeable probe is disclosed in U.S. Pat. No. 6,064,488 in which the elongate hollow structure comprises a tube having slots relieved in the upper and lower surfaces thereof. Filters of sintered metal are welded into the resulting windows to allow gas flowing through a chimney to enter into the optical cavity. The porosity, area and location of the filters in the known arrangements determine the rate at which gas diffuses through the optical cavity. Gas permeable probes of the above kind are used in optical analyzers designed to carry out spectral analysis of gases contained in the optical cavity. Since the gases contained in the optical cavity connected to the gases flowing through the duct or chimney it is possible, using a spectral analyzer, to obtain information on the type of gas that are present in the duct or chimney and their relative concentrations.
Moreover, a gas permeable probe of this kind can also be used to obtain information on various types of dust and dust contents in gas flows such as exhaust streams. This can be done if the pore size of the filter is selected such that the dust of interest can enter into and escape from the optical cavity.
Gas permeable probes of the kind to which the present application relates can be used in gas carrying ducts, especially exhaust ducts of all kinds which operate in a temperature range of e.g. 50° C. to 450° C. Such ducts are, for example, found in power stations, refuse burning plants, in cement works, in association with large furnaces, in steelworks and in gasworks.
In the gas permeable probe known from U.S. Pat. No. 4,560,873 the optical window takes the form of a field lens and it may be provided with a sealing means such as an O-ring to isolate the optical cavity from the rest of the system. A system is provided for continuously supplying a purging gas to a hollow structure provided between the transceiver of the optical analyzer and the lens to keep this hollow structure and the transceiver free of stack gases, i.e. gases flowing through the duct or chimney, and any other contaminated gases. In this way the transmission of light from and to the transceiver will not be undesirably attenuated by unwanted and unknown gases and therefore not give false signals.
However, this system is very wasteful since it requires a continuous supply of purging gas and indeed over a potentially very long period of time. Moreover, the purging gas must be kept free of contaminants, such as dust, lubricating oil and water vapor, because otherwise the optical elements in the transceiver and the above-mentioned lens could still become contaminated with dust, oil, an emulsifier of oil in water or simply by condensation, which would all undesirably affect the quality of the measurement and require periodic cleaning.